Dogs are living longer these days because of the advances of modern veterinary and human medicine. Longer life is a boon to pet-owning families, but it also brings old age problems to Fido and Fancy. Cancer — the dreaded “Big C” — can be a trade-off for those extra years with a loved companion.
The good news is that treatment and cure of animal cancers is proceeding apace: early detection through imaging technology and a range of treatments including radiation therapy, chemotherapy, improved surgical techniques, gene therapy, radioactive beads, and a special diet for the cancer patient are all in the news.
Cancers are malignant neoplasms (growths) that occur when cell-dividing ‘brake’ mechanisms – the genes that keep growth in check – go awry. Causes of these gene malfunctions may be inherited or acquired. Some breeds of dogs seem particularly susceptible to some inherited types of cancer. For example, studies have shown that the English Setter, Chihuahua, Miniature Poodle, and Afghan Hound have a predilection for invasive mammary gland cancers; Boxers, American Staffordshire Terriers, Chinese Shar-Pei, and Boston Terriers have a predisposition to mast cell (deep skin cell) cancers; and Bernese Mountain Dogs are prone to malignant histiocytosis, a cancer of the white blood cells. Unspayed female dogs (and those spayed late in life) are more susceptible to mammary gland cancers, and unneutered male dogs (especially those with a retained testicle) are at risk for testicular cancer. Malignancies are generally diseases of older dogs, so owners should be diligent in their observation and handling of senior pets.
Mixed breed dogs are not immune to hereditary cancers; they can suffer from malignant skin tumors if they inherit a faulty brake gene from one parent.
Acquired cancers arise when the cell mechanisms are disrupted by a virus (although rare in dogs), by irradiation (skin cancers on dogs with short coats and pale skin exposed to excessive sunlight), or by an accumulation of toxic chemicals, either from failure of a cell to dispose of its waste byproducts or from natural or man-made chemicals in the environment.
Body type is also implicated in the development of some types of cancer. Large and giant dogs have a higher incidence of the bone cancer osteosarcoma that is associated with stress on weight-bearing limbs and dogs with dark skin are susceptible to melanomas.
A tumor is any lump or bump, including an abscess, a wart, a hematoma, or a benign or malignant growth. A neoplasm is a tumor that grows as opposed to one that swells; a benign neoplasm is non-cancerous, and a malignant neoplasm is cancer. Cancers can be divided into three general groups: carcinomas that arise from epithelial cells that cover the body (skin) or line internal body surfaces; sarcomas that begin in connective, skeletal, muscular, or reproductive tissues; lymphomas that invade the lymph nodes and the lymphatic system.
The Dog Owner's Home Veterinary Guide by Delbert Carlson DVM and James Griffin MD, has the following information about tumors that can be seen or felt on the body's surface.
Cysts are common in dogs and can occur all over the body.
Sebaceous cysts are non-malignant lumps up to an inch in size filled with a cheesy material called keratin. Sebaceous cysts should be drained or removed to avoid infection.
Warts may occur in older dogs.
Dogs can also get papillomas, which can look like skin tag or a piece of chewing gum stuck on the skin. They are non-malignant, but if they become irritated or start to bleed, they should be removed.
Lipomas are fatty tumors surrounded by a fibrous capsule that isolates their contents from surrounding body fat. These tumors are generally round and smooth and have a fat-like consistency. Lipomas are not painful but can get quite large. They are more common in overweight dogs, especially females.
Hematomas are pockets of blood collected beneath the skin. They can be caused by a blow or contusion and often resolve by themselves.
Ear flap hematomas often occur in dogs that shake their heads frantically because of irritation from ear mites or infection. Large hematomas should be drained and ears should be treated to prevent recurrence.
Histiocytomas occur on the face, feet, and ears of younger dogs. They are dome-shaped, irritated, and painful to the touch. Some histiocytomas disappear on their own and some need to be removed.
Perianal gland tumors primarily affect male dogs. They arise in the anal area; most are benign but can cause pain and get infected.
Skin cancers may resemble the tumors described above; if there is any doubt, a needle aspirate or biopsy should be done. Skin cancers include:
Sebaceous adenomas that arise from the skin's oil-producing glands. These tumors are usually less than an inch long, are light-colored, and look like cauliflower. They should be removed. Cocker Spaniels seem to be affected more than other breeds.
Mast cell tumors are common in older dogs and prevalent in Boxers and Boston Terriers. They occur in older dogs and can be found on the hind legs, lower abdomen, and the foreskin of the penis. About one-third of the mast cell tumors are malignant.
Epidermoid carcinoma is a cauliflower-like tumor or a hard, flat, grayish ulcer that does not heal. Dogs may lick the area and cause hair loss. this tumor usually occurs on the feet and legs and is malignant.
Melanomas are malignant neoplasms that can develop in a pre-existing mole. It is dark in color. If the mole spreads out, bleeds, or becomes elevated above the skin surface, suspect melanoma. Cocker Spaniels, Boston Terriers, and Scottish Terriers are some breeds affected by melanomas.
Other canine cancers include:
Breast tumors are the most common neoplasms in dogs; about half are malignant. A biopsy is necessary to determine the nature of the tumor. Breast cancers can spread, particularly to the lungs. An chest x-ray should be done before surgery to determine if the cancer has metastasized. Any knot or swelling in the mammary glands should be checked immediately.
Osteogenic sarcomas are the most likely bone cancers. This cancer usually strikes middle-aged dogs and affects males more than females. The tumor appears on the long bones of the legs or the flat bones of the ribs. Larger breeds are more susceptible.
Unlike many other diseases, cancer begins without symptoms. There’s no sudden-onset fever or rash or cough, or spell of vomitting or diarrhea to alert the owner that something is wrong. Owners should therefore be vigilant, especially with older dogs and with breeds that have a history of inherited forms of cancer.
Annual checkups that include abdominal palpation, a thorough hands-on exam, and a look in the dog’s mouth are essential for early detection of many tumors, but owners must also check their dogs for any new or changed lumps or bumps and for emergence of the following symptoms1:
Obviously, many of these symptoms are common in other diseases as well; an appointment at the veterinary clinic is necessary to determine whether they are signs of cancer.
Veterinarians use a needle and syringe to extract a few cells from a lump or bump for examination under a microscope. If abnormal cells appear, a biopsy – surgical removal of a piece of tissue for microscopic examination – can be done. Cells from the biopsy tissue are then examined under the microscope and the tumor is assessed as high grade (aggressive), intermediate, or low-grade (slow-growing).
Following discovery of the malignancy and grading its growth rate, veterinarians determine the stage of infection, the potential outcome of the particular type of malignancy, and devise a treatment protocol.
Imaging technology using x-rays, ultrasound, magnetic resonance imaging, and nuclear scintigraphy (scans) are helpful as diagnostic tools and in monitoring the progress of treatment plans.
Ultrasonography enables the veterinarian to see and assess neoplasms that may or may not be externally palpable and to safely direct biopsy instruments for gathering tissue samples for biopsy. A CT (x-ray computed tomography) scan or an MRI may find a brain tumor that does not show up on skull radiographs. X-rays are commonly available in veterinary clinics; ultrasound equipment is becoming more common, but MRI and CT technology is generally not in widespread use.
The most common treatments for cancer are surgical excision, radiation, and tumor-killing drugs. Choice of method depends on the type of tumor, whether it has or is likely to metastasize, and how far it has progressed.
Once the presence of disease has been affirmed, owners and their veterinarians have some decisions to make about treatment potential and quality of life for the dog as well as determining the best treatment choice for the type of malignancy involved. It’s a time of tough decisions as owners consider the dog’s age and general health as well as the cost and potential outcome of prolonged aggressive treatment.
Surgical excision is preferred when the tumor is small, is easily accessible, or is located in a spot that makes it painful. The object of surgery is to remove the entire tumor and leave clean edges that have no cancer cells.
Radiation is useful on cancers of the limbs, for locally invasive tumors such as nasal carcinomas, and for tumors that cannot be completely removed surgically. Success is best when the tumor is small; large cancers are surgically removed before the treatments and radiation is used to stop further growth of any cells missed in the surgery.
Radiation works best on those cancers that are low-grade — the ones that are localized and metastasize late in the course of the disease. It is effective on malignancies on the limbs, chest, face, and mouth, but is not indicated for brain, spinal cord, or intestinal cancers. Once the cancer has spread to internal organs, chemotherapy or a combination of radiation and chemotherapy may be appropriate.
Radiation sterilizes the cancer cells so they cannot divide. Scar tissue may cause the lump to remain, however. The treatment is not painful, but does have some side effects, including hair loss at the tumor site, skin burn similar to sunburn, and occasionally ulcerated areas. The side effects begin towards the end of the treatment period and generally subside in about two weeks.
Chemotherapy, which attacks rapidly-dividing cells, can be used as the primary treatment for cancers such as lymphosarcma or leukemia and in conjunction with surgery or radiation for other types of the disease.
Veterinarians use the same drugs used by human oncologists, but dogs do not generally suffer the side effects of hair loss and nausea that are experienced by human cancer patients. Because chemotherapy in dogs is generally used to prolong life, not cure the cancer, lower doses and fewer drug combinations are used.
Dogs with cancer can become malnourished if their metabolic processes are affected by the disease or if they suffer from nausea, vomiting, or intestinal upsets as a result of chemotherapy treatment. Because optimum nutrition is necessary to fight the disease throughout the treatment period and to maintain optimum health when the treatment is over, Hills Pet Nutrition, the University of Colorado, and the Morris Animal Foundation pooled resources to create a special diet for cancer patients. The resulting formula is available as Hill’s Prescription Diet® Canine n/d.
Dogs with nausea or diarrhea many associate eating with feeling bad and may therefore develop an aversion to food. The following list of feeding suggestions for cancer-stricken dogs is derived from guidelines developed by veterinary oncologists at the University of Pennsylvania.
Still in the research stages, gene therapies include development of medicines to be injected into the tumor to jump start the immune system into battling the cancer and preventing metastasis. The research parallels that done for human cancer treatment and is showing good results in shrinking soft tissue sarcomas with few side effects, according to a report from scientists at Heska Corporation2.
Morris Animal Foundation funds studies researching gene therapies for canine cancer treatment and reports some success in treating osteosarcoma, melanoma, soft tissue sarcomas, and lymphomas3.
The University of Kansas is investigating the use of radioactive beads implanted in the tumor, and researchers at other institutions are looking at ways to shut off tumor growth by cutting off the blood supply to the maverick cells.
The American Kennel Club Canine Health Foundation is awarding grants to discover gene markers for various types of cancers that are known or suspected to be inherited. Once the genes are discovered, tests may be developed to help breeders make decisions that will reduce and hopefully eliminate cancers from their dogs. Money for these grants comes from donations from breeders, breed clubs, and fanciers.
Please remember this article is intended to provide background information to make you a better informed dog owner. We are not medical professionals; we have no knowledge of, or expertise in, diagnosis or other veterinary protocols and have not examined your particular dog. No article, no matter how carefully researched, can replace a medical professional's evaluation of your dog. If you have a concern about your pet's physical condition or a course of treatment, you should immediately contact your veterinarian. Do not hesitate to ask your veterinarian for additional information or for a referral to a specialist for a second opinion.
Please don't waste valuable time by writing to us. We do not answer medical questions.
1. Table 1, cancer chapter by Bruce Madewell DVM, MS, page 409, UC Davis Book of Dogs edited by Mordecai Siegal
2. A report given at the Veterinary Cancer Society meeting, Ft. Collins, Colorado, October 2001. Heska discovers, develops, manufactures and markets companion animal health products, primarily for dogs, cats and horses. For more information, visit the Heska website at www.heska.com.
3. AnimalNews Volume II, 2001. AnimalNews is a publication of Morris Animal Foundation, a nonprofit organization that improves the health and well-being of companion animals and wildlife. For more information, visit the Morris Animal Foundation at www.morrisanimalfoundation.com.
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